“Comet,
an astronomical body of small mass, moving about the sun in a more or less
elongated orbit. When it is sufficiently close to the sun, icy materials in the
main mass, or nucleus, are vaporized by sunlight and form a hazy envelope of
gases and finely divided dust particles. The presence of this diffuse envelope,
which tends to obscure the nucleus, is a characteristic and defying feature. The
dust and gases may then form one or more tails, which are always directed more
or less away from the sun. The term comet derives from a Greek word meaning
“long-haired,” which is descriptive of the following appearance of the tail
of a bright comet.

Comets were
formerly of interest to astronomers principally because of problems connected
with their motions. They are now recognized as representing probably the
least-changed available sample of the primordial material from which the sun and
planets were formed. Thus they are studied today in order to understand their
physical and chemical characteristics.

Properties

Orbits.
Comets are generally classified, according to the size and shape of their
orbits, into short period and long period objects. The former move in elliptical
orbits that require less than 200 years per revolution. Some of them have been
observed repeatedly for many revolutions as they pass the earth and sun. The
long-period comets move in much larger and more elongated orbits that are nearly
parabolic in shape. Such comets come to the vicinity of the sun only at
intervals of thousands or millions of years. A recent catalog of orbits listed
135 short-period comets and 613 that moved in nearly parabolic orbits.

The great
majority of short-period comets have periods between 3 and 9 years long. Of
these, over 120 comets known as the Jupiter family have aphelia that are located
close to the orbit of Jupiter. (Aphelion is the orbital point most distant from
the sun.) Essentially all the remaining comets of short period appear to have
passed close to Jupiter at one time or another. Although associations of similar
comet families with other planets have been suggested, their reality is
doubtful. The probability of the dominance of Jupiter in shaping the smaller
orbits seems overwhelming.

The orbit
planes of short-period comets also show a concentration toward the average plane
of motion of the major planets. Practically all these comets revolve about the
sun in the same direction, as do the planets. Comet Halley, which revolves in
the retrograde orbit inclined some 18 degrees to the plane of the earth’s
orbit, is one noteworthy exception.

The orbits of
the long-period comets, on the other hand, reach far beyond the domain of the
major planets and out toward interstellar space. Such orbits are distributed
essentially at random with respect to the inclination of the orbit planes to the
plane of the earth’s orbit around the sun. The apparent distribution of
orientations of the long axes of the comets orbits in their planes is more
complex, being affected by such factors as the motion of the sun in the Milky
Way Galaxy and effects of observing conditions on comet discoveries.

Storage
Clouds of Comets. From a study of the sizes and shapes of the
best-determined nearly parabolic orbits, freed from the disturbing effects of
the major planets, the Dutch astronomer J. Oort concluded in 1950 that fresh
comets must come from a vast storage region, later called the Oort cloud, at the
far outer edges of the solar system. He supposed that large numbers of comets
move slowly in a sort of deep freeze in this region, continually stirred by
gravitational disturbances produced by passing stars. Occasionally a comet would
be so deflected as to be sent into the inner solar system, where it could be
observed. To account for the frequency with which such comets were seen, Oort
calculated that there must be some 100 billion comets in this distant, nearly
spherical, cloud. Later calculations showed that galactic tides and giant
interstellar clouds, as well as passing stars, disturb the motion of the distant
comets. Their number must even be larger than Oort supposed, and their total
mass is likely to be at least several times the mass of the earth.

Many comets
would have been lost from distant cloud over time to interstellar space.
Further, there are difficulties in capturing comets from nearly parabolic obits
into small elliptic ones efficiently enough to match the observed numbers of
short-period comets. These, and some other considerations, led to the idea of an
inner storage cloud. Such a cloud is thought to extend from just beyond the
planets to merge into the distant Oort cloud. The inner portion probably is
substantially flattened to the average plane of the planetary system. Almost
certainly the inner storage cloud contains a mass of comets substantially larger
than that of the distant Oort cloud. On relatively rare occasions the inner
cloud might be disturbed by the passage of a star unusually close to the sun. As
a consequence, showers of comets could be sent into the inner solar system, some
of them to be captured by Jupiter and the large planets into short period
orbits. Others would be thrown outward to repopulate the Oort cloud.

As comets
repeatedly penetrate the part of solar system closest to the sun, their orbits
are gradually changed by the gravitational attraction of the planets, especially
Jupiter. Some are ejected from the solar system into interstellar space. Some
return to the Oort cloud. And some, especially those that move in planes close
to those of the principal planets, may be deflected into small, distinctly
elliptical orbits like those of the short-period comets.

Formation
of Comets. Comets probably formed in the outer portions of the solar nebula,
either in the vicinity of the outermost planets or perhaps as far from the sun
as the proposed massive inner cloud of comets. Some old grains that condensed in
dense cold parts of interstellar clouds may have survived the formation of he
solar nebula to be incorporated virtually unchanged into the nuclei of comets.

The early
history of the solar system is very hard to decipher from the properties of the
highly metamorphosed planets and asteroids that have long stayed close to the
sun. If comets were formed at the same time as the planets but in the cold outer
portions of the solar nebula, it seems very probable that some of the record of
those early times is locked in the present chemical and physical structure of
the comets. Thus they have become targets for intensive scientific investigation
from earth and from spacecraft.”

Elizabeth Roemer

University of Arizona

Bibliography

Brandt, John C., and Chapman, Robert D., Introduction to
Comets (Cambridge 1982)

“Comets also
are comic debris, probably planetesimals that originally resided in the vicinity
of the orbits of Uranus and Neptune rather than in the warmer regions of the
asteroid belt. Thus, the nuclei of the comets are icy balls of frozen water,
methane, and ammonia, mixed with small pieces of rock and dust, rather than the
largely volatile-free stones and irons that typify asteroids. In the most
popular theory, ice planetesimals in the primitive solar nebula that wandered
close to Uranus or Neptune but not close enough to be captured by them were
flung to great distances from the Sun, some to be lost from the solar system
while others populated what was to become a great cloud of cometary bodies,
perhaps 10 trillion in number. Such a cloud was first hypothesized by the Dutch
astronomer Jan Hendrik Oort.

In the
original version of the theory, the Oort cloud extends tens of thousands of
times farther from the Sun than the Earth, a significant fraction of the way to
the nearest stars. Random encounters with passing stars would periodically throw
some of the comets into new orbits, plunging them back toward the heart of the
solar system. As a comet nears the Sun, the ice begins to evaporate, loosening
the trapped dust and forming a large coma that completely surrounds the small
nucleus, which is the ultimate source of all material. The solar wind blows back
the evaporating gas into an ion tail, and radiation pressure pushes back the
small particle solids into a dust tail. Each solid particle is an independently
orbiting satellite of the Sun, and the accumulation of countless such passages
by many comets contributes to the total quantity of dust particles and
micrometoroids found in interplanetary space.

The total mass
contained in all the comets is highly uncertain. Modern estimates range from 1
to 100 Earth masses. Part of the uncertainty concerns the reality of a
hypothesized massive “inner Oort Cloud” or“Kuiper belt” (if the distribution is flatten) of the comets that
would exist at distances from the Sun 40 to 10,000 times that of the orbit of
Earth. At such locations, the comets would not be much perturbed by the typical
passing stars nor by the gravity of the planets of the solar system, and the
comets cloud reside in the inner cloud or belt for long periods of time without
detection. It has been speculated, however, that a rare close passage by another
star (possibly an undetected companion of the Sun) may send a shower of such
comets streaming toward the inner solar system. If enough large cometary nuclei
in such showers happen to strike the Earth, the clouds of dust ash that they
would raise might be sufficient to trigger mass biological extinctions. As event
of this kind appears especially promising for explaining the relatively sudden
disappearance from Earth.”